First disable all your plugins while upgrading and revert to the default theme of WordPress.

Now perform the auto-install upgrade to WP version 2.8.5.You’d have a very smooth upgrade.Now try to enable all your desired plugins.Now you may find that some plugins are still causing the above said error message.

Here’s the fix for it:

Open your FTP client, and open wp-config.php file in your WordPress installed directory.Add the following line:

define('WP_MEMORY_LIMIT', '64M');

This would increase the amount of memory that is being consumed by the PHP script.

A fix for the Trackback Denial-of-Service attack that is currently being seen.

Removal of areas within the code where php code in variables was evaluated.

Switched the file upload functionality to be whitelisted for all users including Admins.